The present invention relates generally to semiconductor device manufacturing and, more particularly, to a method and system for partial solution replacement in recyclable persulfuric acid cleaning systems.
Wafer cleaning in integrated circuit device manufacturing involves the removal of photoresist residue, fine particles, metal, oxide films, and other such materials formed on the surface of a substrate. Typically, a quantity of concentrated sulfuric acid (H2SO4) is mixed with a quantity of hydrogen peroxide (H2O2) to produce a solution also known as SPM (Sulfuric Peroxide Mixture), or “piranha” clean (H2SO5). In addition, peroxydisulfuric acid (also known as Marshall's acid) may also be produced by electrolyzing an aqueous solution containing sulfate ions in an electrolysis reactor. Peroxydisulfuric acid (H2S2O8) is a powerful oxidizing agent that exhibits a high cleaning performance and is an effective solution for cleaning semiconductor wafers.
Where persulfuric acid is produced by the combination of concentrated sulfuric acid and hydrogen peroxide, the solution loses its oxidizing power as it self-decomposes. As a result, a continuous supply of hydrogen peroxide solution is necessary to compensate for this decomposition. Moreover, the high concentration sulfuric acid has to be replaced once the sulfuric acid concentration in the solution decreases below a certain level. Because SPM is diluted by the water content of the hydrogen peroxide solution, it becomes difficult to maintain a constant liquid composition. Thus, the solution has to be fully discarded and replaced, for example, at prescribed time intervals or following processing of a certain number of wafer batches. These conditions make it difficult to maintain consistent cleaning performance, as well as require a large volume of chemicals to be stored.
One way to prevent dilution of SPM is to blow ozone (O3) into concentrated sulfuric acid. While this leads to a longer solution replacement cycle, the technique is generally inferior in cleaning performance with respect to using hydrogen peroxide. Furthermore, regardless of whether concentrated sulfuric acid is mixed with hydrogen peroxide or with ozone, ultimately the concentration of the resulting SPM is limited. Correspondingly, cleaning performance is also limited.
More recently, recyclable sulfuric acid cleaning systems have been introduced that can largely decrease the sulfuric acid consumption through a persulfate ion recycling process which produces persulfate ions electrochemically from an aqueous sulfuric acid solution while the sulfuric acid is recycled. Generally, these recyclable sulfuric acid cleaning systems include a cleaning vessel that cleans wafers using a persulfuric acid solution as cleaning fluid, an electrolytic reactor that regenerates the persulfuric acid solution by performing electrolytic reaction to produce persulfate ions from sulfate ions contained in the solution, and one or more fluid lines that circulate the persulfuric acid solution between the cleaning apparatus and the electrolytic reaction apparatus.
A recyclable sulfuric acid cleaning system may allow for bath life extension by eliminating decomposing hydrogen peroxide from the resist strip bath and providing a continuous replenishment of the active species. At some point, however, the continued use of the recyclable sulfuric acid cleaning system without bath replacement may result in electrode passivation or coating, which reduces the DC current produced in the electrolytic reactor. Moreover, compensation for this effect by increasing the operating current of the device may cause precipitation of sulfur on the electrodes, which in turn can result in either down time for cleaning the electrodes or the use of expensive replacement electrodes.